Interpretive Summary: Experiments reported here were initiated to assess growth response of highbush blueberry to composts derived from diverse feedstocks and to link the response to compost chemical characteristics. Since the pH of the composts tested was high for blueberry (> 6.5), we also developed a reliable method for compost acidification and evaluated the potential of using acidified compost in the crop. Manures and plant materials that contained high amounts of N were not suitable for blueberry because of associated characteristics of high pH, EC, and K. A high-N crop residue (i.e., mint) was also poorly suited to blueberry. Composts derived from municipal yard debris, leaves from street sweepings, or dewatered municipal biosolids composted with conifer bark appear promising for blueberry and need to be further evaluated as potential materials to replace sawdust in field-scale production systems. Yard debris composts are likely the best candidate for use by growers because they have relatively consistent chemical analyses, can be acidified to pH 5 with a moderate rate of elemental S, and composters have the ability to control particle size by screening. However, yard debris composts from more arid regions such as southern California may not be suitable for blueberry because of higher cation analyses (may result in nutrient imbalances) and high EC. Bark:biosolids composts had near-ideal chemical properties for blueberry. They were the only composts that supplied low levels of K (< 5 g kg-1). Most, but not all dewatered municipal biosolids will be low in K because K follows the liquid stream at wastewater treatment plants. Cation analyses of biosolids will likely be higher in regions with “hard” water. The major limitation to use of biosolids compost is public perception. Compost analyses and plant growth response was also favorable for leaves collected from street sweepings, but availability of this material is more limited than yard debris. Leaves collected from some urban areas may also have unacceptably high levels of Zn, Cu, and Pb and therefore should be tested.

Technical Abstract:
Recent development of markets for blueberry (Vaccinium corymbosum L.) produced under Organic certification has stimulated interest in production of composts specifically tailored to its edaphic requirements. Blueberry is a calcifuge (acid-loving) plant that responds favorably to mulching and incorporation of organic matter into soil. This chapter reports data from initial screening studies conducted in western Oregon USA to (i) assess growth response of blueberry to composts derived from diverse feedstocks, mixed into the soil at high rates (30% v/v in pots), and (ii) to link blueberry response to compost chemical characteristics. Acidic pH (< 5.5) was confirmed as the most important characteristic needed in a custom compost for blueberry. Composts with pH < 6 were rare, so we developed a testing protocol to quantify the pH buffering capacity of compost (CBC) and the quantity of acidity needed to reduce compost pH to 5.0. Median compost buffering capacity (n = 36) was 0.20 mol H+/kg/pH unit and median elemental S addition required for acidification to pH 5 was 8 g/kg, assuming full reaction of the S to H+. In compost acidified with elemental S to a final pH of 5 to 6, EC was increased 1.6 fold, accompanied by increased solubility for P, K, Na (1.3-1.6x); P, Ca, and Mg (3.2-3.6x), SO4-S (5.2x). Blueberry plants accumulated K supplied by compost, accompanied by reduced plant uptake of Mg, reduced growth, and possible Mg deficiency. Compost acidification to < pH 6 improved blueberry plant growth and Mg uptake. We conclude that compost can be used to increase soil organic matter for blueberry, but that compost N must be limited to low analysis values (total N < 20 g/kg) in order to avoid problems with high pH, EC, and excess K. Because all compost feedstocks that met USDA-Organic certification requirements needed acidification to reach the desired pH level for blueberry (< 5.5), future research should focus on economical, safe, and reliable methods for compost acidification that are acceptable under Organic certification rules.